
26
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
5.1.4
PLL Clock – clk
PLL
The PLL clock allows the fast peripherals to be clocked directly from a 64/32MHz clock. A 16MHz clock is also derived for the
CPU.
5.1.5
ADC Clock – clk
ADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order to reduce noise
generated by digital circuitry. This gives more accurate ADC conversion results.
5.2
Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as illustrated in
Table 5-1. The clock from the
selected source is input to the AVR clock generator, and routed to the appropriate modules.
Notes: 1.
For all fuses “1” means unprogrammed while “0” means programmed.
2.
Ext Osc: External oscillator
3.
RC Osc: Internal RC oscillator
4.
Ext Clk: External clock input
The various choices for each clocking option is given in the following sections. When the CPU wakes up from power-down or
power-save, the selected clock source is used to time the start-up, ensuring stable oscillator operation before instruction
execution starts. When the CPU starts from reset, there is an additional delay allowing the power to reach a stable level before
starting normal operation. The watchdog oscillator is used for timing this real-time part of the start-up time. The number of WDT
oscillator cycles used for each time-out is shown in
Table 5-2 on page 26. The frequency of the Watchdog Oscillator is voltage
Table 5-1.
Device Clocking Options Select(1) Device Clocking Option
System
Clock
PLL Input
CKSEL3..0
External crystal/ceramic resonator
Ext Osc
RC Osc
1111 - 1000
PLL output divided by 4: 16MHz / PLL driven by external crystal/ceramic
resonator
Ext Osc
0100
PLL output divided by 4: 16MHz / PLL driven by external crystal/ceramic
resonator
PLL / 4
Ext Osc
0101
Reserved
N/A
0110
Reserved
N/A
0111
PLL output divided by 4: 16MHz
PLL / 4
RC Osc
0011
Calibrated internal RC oscillator
RC Osc
0010
PLL output divided by 4: 16MHz/PLL driven by external clock
PLL / 4
Ext Clk
0001
External clock
Ext Clk
RC Osc
0000
Table 5-2.
Number of Watchdog Oscillator Cycles
Typ Time-out (VCC = 5.0V)
Typ Time-out (VCC = 3.0V)
Number of Cycles
4.1ms
4.3ms
4K (4,096)
65ms
69ms
64K (65,536)